Negative ion photoelectron spectroscopy provides a means of obtaining vibrational data for atoms and small molecules ""chemisorbed"" on size-selected metal clusters. In the present study, $Nb_{3}O^{-}$, $Nb_{4}O^{-}$ and $Nb_{4}CO^{-}$ were prepared in a flowing afterglow ion-molecule reactor equipped with a metal cathode cluster source. The 488 nm photoelectron spectrum of the mass-selected $Nb_{3}O^{-}$ anions shows a vertical transition to the ground state of neutral $Nb_{3}O^{-}$, with weak progressions in the $Nb_{3}-O$ stretching (710$\pm$$20 cm^{-1}$ in $Nb_{3}O$) and $Nb_{3}$ bending (320$\pm$$15 cm^{-1}$ in both $Nb_{3}O$ and $Nb_{3}O^{-}$) vibrational modes. These results indicate that the $Nb_{3}O^{-}$ anion, like $Nb_{3}O$ and $Nb_{3}O^{-1}$, has a planar $C_{3}$, structure with the O atom bridging two Nb atoms. The $Nb_{4}O^{-}$ spectrum shows resolved transitions to the ground state of $Nb_{4}O$ and to an excited electronic state lying 3050$\pm$$20 cm^{-1}$ higher in energy. In analogy with $Nb_{3}O$ results, the 670$\pm$$20 cm^{-1}$ frequencies observed in the ground and excited states, respectively, to a bending mode of the metal cluster. The electron affinities of $Nb_{3}O$ and $Nb_{4}O$ are 1.402 and 1.178 ($\pm$0.006) cV, respectively. Preliminary, ongoing studies of mass selected $Nb_{4}CO^{-}$ anions prepared under a variety of source conditions thus far suggest the presence of two isomers, one with a greatly weakened but intact CO bond as indicated by a very low CO stretching frequency of about $1300 cm^{-1}$, and the other with the dissociated C and O atoms bound separately to the niobium cluster.